1. Field of the Invention
The present invention relates to a method and a device for measuring pulmonary nodule volume using a dual-energy subtraction image.
2. Description of the Related Art
Lung cancer is the most common cancer and the leading cause of cancer deaths around the world. More than one million three hundred thousand people die of lung cancer around the world every year. In South Korea, more than twelve thousand people have died of lung cancer in 2002, and the death rate from lung cancer has been reported to be the highest of all. A pulmonary nodule is commonly observed on a chest radiograph and may be an inflammatory granuloma, benign tumor, or malignant tumor (lung cancer). Accordingly, it is important to determine whether the lesion is benign or malignant in addition to detecting the pulmonary nodule for early lung cancer diagnosis and rapid curative action. However, it is difficult even for medical experts to determine whether a pulmonary nodule is benign or malignant. The diagnostic accuracy of a pulmonary nodule using a computed tomography (CT) is about ⅔. Accordingly, a biopsy is commonly used to determine whether a pulmonary nodule is benign or malignant.
In general, a solitary pulmonary nodule is a round or oval, sharply defined lung lesion and is not accompanied with atelectasis or pneumonia. On the other hand, 70% of lung cancer is characterized by a solitary pulmonary nodule, and about 20% of the solitary pulmonary nodule observed on the x-ray radiograph is finally diagnosed as the lung cancer. Radiographically, the most important thing for determining whether the pulmonary nodule is benign or malignant is a volume doubling time of the pulmonary nodule. In general, the volume doubling time of a malignant nodule is significantly less than that of a benign nodule. According to research, it is known that the volume doubling time of a malignant nodule is within about 200 days, and the volume doubling time of a benign nodule is greater than 4 years. Therefore, to accurately determine whether a pulmonary nodule is benign or malignant, a follow up period of more than 1 to 2 years is needed. However, it is very important to accurately estimate the volume doubling time of the pulmonary nodule during the follow up period of several months after the pulmonary nodule is observed so as to alleviate the burden of patients or to avoid possibility of worsening.
In general, the volume doubling time is calculated by manually or automatically measuring the pulmonary nodule volume seen on a chest x-ray radiograph or CT radiograph. When the pulmonary nodule volume is required to be measured using the chest x-ray radiograph, the area of the pulmonary nodule projected to a 2-dimensional screen is measured, and then the 3-dimensional volume is analogized from the area of the pulmonary nodule. However, when the shape of the pulmonary nodule is not a sphere, it becomes difficult to accurately measure the volume of the pulmonary nodule. In addition, ribs may overlap the pulmonary nodule on the chest x-ray radiograph to deteriorate detection sensitivity of the pulmonary nodule. When using a CT radiograph, the accuracy of measuring the pulmonary nodule volume is relatively high, but the volume measurement error is still about 10%. When CT radiography is used, the volume measurement error is mainly caused by imperfect extraction of the pulmonary nodule border.
Although the method of measuring the pulmonary nodule volume using a chest x-ray radiograph or CT radiograph exists, a method of measuring the pulmonary nodule using dual-energy subtraction image has been developed. The dual-energy subtraction imaging technique selectively removes the x-ray absorption effect of bones or soft tissues by using x-ray attenuation difference between the bones and the soft tissues from two x-ray images taken by x-rays of which energy bands are different. Accordingly, in dual-energy subtraction images, rib shadows seen on the simple x-ray radiograph are removed to clearly reveal the pulmonary nodule and improve the volume measurement accuracy.